A First for Human Vaccinology: GMP Compliant Radiation Attenuation of Plasmodium falciparum Sporozoites for Production of a Vaccine Against Malaria

Abstract

Ionizing radiation (UV, X-ray and ɣ) administered at an appropriate dose to pathogenic organisms can prevent replication while preserving metabolic activity. We have established the GMP process for attenuation by ionizing radiation of the Plasmodium falciparum (Pf) sporozoites (SPZ) in Sanaria® PfSPZ Vaccine, a protective vaccine against malaria. Mosquitoes raised and infected aseptically with Pf were transferred into infected mosquito transport containers (IMTC) and ɣ-irradiated using a 60Co source. PfSPZ were then extracted, purified, vialed, and cryopreserved. To establish the appropriate radiation conditions, the irradiation field inside the IMTCs was mapped using radiochromic film and alanine transfer dosimeters. Dosimeters were irradiated for times calculated to provide 120-170 Gy at the minimum dose location inside the IMTC and regression analysis was used to determine the time required to achieve a lower 95% confidence interval for 150 Gy. A formula incorporating the half-life of 60Co was then used to construct tables of irradiation times for each calendar day. From the mapping studies, formulae were derived to estimate the minimum and maximum doses of irradiation received inside the IMTC from a reference dosimeter mounted on the outside wall. For PfSPZ Vaccine manufacture a dose of 150 Gy was targeted for each irradiation event, a dose known to completely attenuate PfSPZ. The reference dosimeters were processed by the National Institute of Standards and Technology. There have been 587 irradiation events to produce PfSPZ Vaccine during 13 years which generated multiple lots released for pre-clinical studies and clinical trials. The estimated doses at the minimum dose location (mean 154.3 ± 1.77 Gy; range 150.0-159.3 Gy), and maximum dose location (mean 166.3 ± 3.65 Gy, range 155.7 to 175.3 Gy), in IMTCs were normally distributed. Overall dose uniformity was 1.078 ± 0.012. There was no siginifcant change in measured dose over 13 years. As of January 2022, 21 clinical trials of PfSPZ Vaccine have been conducted, with 1,740 volunteers aged 5 months to 61 years receiving 5,648 doses of PfSPZ Vaccine totalling >5.3 billion PfSPZ administered. There have been no breakthrough infections, confirming the consistency and robustness of the radiation attenuation process.

Apicomplexan parasites are attenuated by low-energy electron irradiation in an automated microfluidic system and protect against infection with Toxoplasma gondii

“…The advantages of ionizing radiation include the short process time and the applicability to various parasite strains or isolates. Irradiation-attenuated parasites have been shown to induce protective immune responses in several animal species (Hiramoto et al 2002 ; James et al 2022 ; Jenkins et al 2004 ; Oneko et al 2021 ; Zorgi et al 2011 ). On the other hand, the relatively high doses needed for irradiation of many parasites require complex irradiation facilities.…”

“…The major effect of ionizing radiation on organisms is the damage of nucleic acids while protein structures remain largely intact (Alsharifi and Müllbacher 2010 ; Hutchinson 1985 ). There are several reports on experimental parasitic vaccines generated by ionizing irradiation, including T. gondii (Hafez et al 2020 ; Hiramoto et al 2002 ; Zorgi et al 2011 ), C. parvum (Jenkins et al 2004 ), Plasmodium falciparum (Hoffman et al 2002 ; James et al 2022 ; Oneko et al 2021 ), or hookworm larvae (Roh 1968 ; Xiao et al 1998 ).…”

Advancements and Challenges in Developing Malaria Vaccines: Targeting Multiple Stages of the Parasite Life Cycle

“…RAS vaccines involve exposing sporozoites to an optimal dose of ionizing radiation, which can halt parasite replication while keeping metabolic activity in check. 100 These attenuated sporozoites are capable of invading hepatocytes but are unable to replicate and develop into erythrocyte-infectious merozoites due to significant DNA damage and reduced expression of DNA repair genes. 101 The use of RAS immunization was first demonstrated in a study by Nussenzweig and colleagues in 1967, where RASimmunized mice were protected against malaria.…”